See also

At present, there are 3 load balancer scheduler algorithms available
for use: Request Counting, Weighted Traffic Counting and Pending Request
Counting. These are controlled via the lbmethod value of
the Balancer definition. See the ProxyPass
directive for more information, especially regarding how to
configure the Balancer and BalancerMembers.

The balancer supports stickyness. When a request is proxied
to some back-end, then all following requests from the same user
should be proxied to the same back-end. Many load balancers implement
this feature via a table that maps client IP addresses to back-ends.
This approach is transparent to clients and back-ends, but suffers
from some problems: unequal load distribution if clients are themselves
hidden behind proxies, stickyness errors when a client uses a dynamic
IP address that changes during a session and loss of stickyness, if the
mapping table overflows.

The module mod_proxy_balancer implements stickyness
on top of two alternative means: cookies and URL encoding. Providing the
cookie can be either done by the back-end or by the Apache web server
itself. The URL encoding is usually done on the back-end.

This is set to 1 if the session route does not match the
worker route (BALANCER_SESSION_ROUTE != BALANCER_WORKER_ROUTE) or the
session does not yet have an established route. This can be used to
determine when/if the client needs to be sent an updated route
when sticky sessions are used.

This module requires the service of
mod_status.
Balancer manager enables dynamic update of balancer
members. You can use balancer manager to change the balance
factor of a particular member, or put it in the off line
mode.

You can now access load balancer manager by using a Web browser
to access the page
http://your.server.name/balancer-manager. Please note
that only Balancers defined outside of <Location ...>
containers can be dynamically controlled by the Manager.

When using cookie based stickyness, you need to configure the
name of the cookie that contains the information about which back-end
to use. This is done via the stickysession attribute added
to either ProxyPass or
ProxySet. The name of
the cookie is case-sensitive. The balancer extracts the value of the
cookie and looks for a member worker with route equal
to that value. The route must also be set in either
ProxyPass or
ProxySet. The cookie can either
be set by the back-end, or as shown in the above
example by the Apache web server itself.

Some back-ends use a slightly different form of stickyness cookie,
for instance Apache Tomcat. Tomcat adds the name of the Tomcat instance
to the end of its session id cookie, separated with a dot (.)
from the session id. Thus if the Apache web server finds a dot in the value
of the stickyness cookie, it only uses the part behind the dot to search
for the route. In order to let Tomcat know about its instance name, you
need to set the attribute jvmRoute inside the Tomcat
configuration file conf/server.xml to the value of the
route of the worker that connects to the respective Tomcat.
The name of the session cookie used by Tomcat (and more generally by Java
web applications based on servlets) is JSESSIONID
(upper case) but can be configured to something else.

The second way of implementing stickyness is URL encoding.
The web server searches for a query parameter in the URL of the request.
The name of the parameter is specified again using stickysession.
The value of the parameter is used to lookup a member worker with route
equal to that value. Since it is not easy to extract and manipulate all
URL links contained in responses, generally the work of adding the parameters
to each link is done by the back-end generating the content.
In some cases it might be feasible doing
this via the web server using mod_substitute or
mod_sed. This can have negative impact on performance though.

The Java standards implement URL encoding slightly different. They use
a path info appended to the URL using a semicolon (;)
as the separator and add the session id behind. As in the cookie case,
Apache Tomcat can include the configured jvmRoute in this path
info. To let Apache find this sort of path info, you neet to set
scolonpathdelim to On in
ProxyPass or
ProxySet.

Finally you can support cookies and URL encoding at the same time, by
configuring the name of the cookie and the name of the URL parameter
separated by a vertical bar (|) as in the following example:

If you experience stickyness errors, e.g. users lose their
application sessions and need to login again, you first want to
check whether this is because the back-ends are sometimes unavailable
or whether your configuration is wrong. To find out about possible
stability problems with the back-ends, check your Apache error log
for proxy error messages.

To verify your configuration, first check, whether the stickyness
is based on a cookie or on URL encoding. Next step would be logging
the appropriate data in the access log by using an enhanced
LogFormat.
The following fields are useful:

%{MYCOOKIE}C

The value contained in the cookie with name MYCOOKIE.
The name should be the same given in the stickysession
attribute.

%{Set-Cookie}o

This logs any cookie set by the back-end. You can track,
whether the back-end sets the session cookie you expect, and
to which value it is set.

%{BALANCER_SESSION_STICKY}e

The name of the cookie or request parameter used
to lookup the routing information.

%{BALANCER_SESSION_ROUTE}e

The route information found in the request.

%{BALANCER_WORKER_ROUTE}e

The route of the worker chosen.

%{BALANCER_ROUTE_CHANGED}e

Set to 1 if the route in the request
is different from the route of the worker, i.e.
the request couldn't be handled sticky.

Common reasons for loss of session are session timeouts,
which are usually configurable on the back-end server.

The balancer also logs detailed information about handling
stickyness to the error log, if the log level is set to
debug or higher. This is an easy way to
troubleshoot stickyness problems, but the log volume might
be to high for production servers under high load.

Notice:This is not a Q&A section. Comments placed here should be pointed towards suggestions on improving the documentation or server, and may be removed again by our moderators if they are either implemented or considered invalid/off-topic. Questions on how to manage the Apache HTTP Server should be directed at either our IRC channel, #httpd, on Freenode, or sent to our mailing lists.